1. Field of the Invention
This invention relates to a process for removing relatively small quantities of sulfur dioxide gas from gaseous mixtures. More particularly, the present invention relates to processes for removing trace quantities of sulfur dioxide by subjecting a gaseous mixture containing such trace quantities to a scrubbing action with an alkali metal sulphite to yield a condensed phase aqueous mixture containing sulfite compounds, then extracting the sulfite ions from the pregnant aqueos scrubbing solution by contact thereof with an immiscible organic reactant to regenerate the scrubbing solution, followed finally by the release of concentrated sulfur dioxide gas from the extraction step organic phase in a form which is useful, per se, or can be easily converted to elemental sulfur.
2. Brief Description of the Prior Art
In my co-pending application Ser. No. 499,582, now U.S. Pat. No. 3,984,529 I review various prior efforts to provide economical and efficient processes for removing sulfur dioxide gas from gaseous mixtures such as stack gases or the like. Removal of sulfur dioxide is desirable in order to obviate or reduce atmospheric pollution, and also in order to avoid the loss of the economic value represented by the recovered sulfur or sulfur compounds constituting potential end products of the recovery process.
In my co-pending application, to which reference is made, I described certain proposed procedures for removing small or trace quantities of sulfur dioxide from gaseous effluents or mixtures of the type described, which processes or methods generally entail the steps of initially contacting the gaseous mixture which contains the sulfur dioxide with an aqueous solution containing a removal reactant so as to form a condensed phase in which the removed sulfur dioxide is combined chemically with the removal reactant in the form of sulfite or hydrosulfite compounds in aqueous solution. The removal reactants contemplated for use in the processes described in the co-pending application include, inter alia, alkali metal hydroxides, and salts of weak acids and alkali metal hydroxides.
After formation of the pregnant scrubbing solution which contains compounds including combined sulfite ions derived from the extracted SO.sub.2 gas, the pregnant scrubbing solution is contacted with an organic liquid phase which includes, as an active component, certain nitrogen-containing water-immiscible organic compounds which have certain prescribed properties. Among such compounds which are effective and are preferred at this point in the process are various types of long chain alkyl amines which contain from about 12 to about 45 carbon atoms, and have a solubility in water of less than 0.2 gm/100 gms of water at 25.degree. C. The result of contacting the pregnant scrubbing solution with the described amine-containing organic liquid phase is to transfer the sulfite ions from the pregnant aqueous scrubbing solution to the organic phase as a result of chemical combination of sulfite ions with the long chain alkyl amines. A concomitant result of the described contact is the regeneration of the scrubbing solution containing the selected removal reactant so that it can be recycled to the scrubbing zone, and used for contacting the gaseous mixture from which additional SO.sub.2 is to be subsequently removed.
The organic phase developed upon contact of the long chain amine with the pregnant scrubbing solution contains long chain amine sulfites, and is contacted with hydrogen sulfide so as to reduce the sulfur-containing salts therein to elemental sulfur, and concurrently regenerate the long chain amine. The amine can then be reutilized for contacting additional sulfite-containing pregnant scrubbing solution from the scrubbing zone for the purpose of repeating the transfer of sulfite ions into the organic phase.
In Wiewiorowski U.S. Pat. No. 3,633,339, a process is prescribed for removing sulfur dioxide from gases, and initially entails contacting the gas containing the sulfur dioxide with an aqueous solution of ammonium phosphate. The pH of the thus constituted scrubbing solution is indicated by the patentee to be from about 3.0 to about 5.0, with the best range being between 3.5 and 4.5. The pregnant scrubbing solution contains ammonium hydrosulfite and ammonium phosphate. This pregnant scrubbing solution is then contacted with an organic extractant to extract sulfite ions into the organic phase. Subsequently, the organic phase is heated and subjected to stripping to yield concentrated sulfur dioxide gas therefrom.
Several problems characterize the Wiewiorowski process which make it of less than optimum efficiency in the removal of the sulfur dioxide gas from the polluted gas stream treated, and in the ultimate recovery of sulfur dioxde in concentrated form. The ammonium phosphate utilized as the active agent in the scrubbing solution has a pH sufficiently low, in contrast to certain other types of scrubbing agents, to place an undesirable limit on the capacity of the scrubbing solution to combine with and remove SO.sub.2. In other words, the relatively low pH of the ammonium phosphate scrubbing solution reduces the capacity of the scrubbing agent to combine with, and carry out of the contaminated gaseous stream therewith, large quantities of SO.sub.2 per pass of the scrubbing solution.
Further, though it is not a major feature of objection to the ammonium phosphate scrubbing process, the use of ammonium salt of this type tends to cause the formation of an undesirable "plume" at the top of the flue gas stack--an obviously undesirable aspect in terms of real, as well as apparent, atmospheric pollution. The plume results from finely divided ammonium salts apparently formed in the gas phase reactions.
After development of the pregnant scrubbing solution which comprises an aqueous solution of an ammonium hydrosulfite and ammonium phosphate salts, less than optimum selectivity is attained in the transfer of the sulfite ions to a given quantity or volume of the organic transfer reactant with which the pregnant scrubbing solution is contacted for sulfite ion extraction purposes. Thus, some of the amine compounds constituting the active extractive components in the organic phase at this step of the process will, indeed, combine with sulfite ions to effect the desired ion transfer preparatory to subsequent release of concentrated SO.sub.2. A part of the amine compounds in the organic phase will, however, combine with phosphate ions, and will be rendered inaccessible to sulfite ions, and therefore ineffective for abstracting or carrying over into the organic phase, the sulfite ions constitutng the intermediate source of the ultimate sulfur dioxide product.
In other words, transfer of phosphate ions into the organic layer at this stage of the process effectively ties up a part of the amine compounds so as to render them inactive toward the sulfite ion transfer reaction. Therefore, a substantially greater total quantity of amine must be used, and more power is consumed in circulating the organic phase and its contained transfer reactant in the course of the process.
Further, the phospate ions which are introduced into the organic layer in this fashion must be separately removed (from the sulfite ions) to avoid their build-up and to regenerate the equivalent quantity of free amine compounds for reuse in the transfer reaction, and the result is a net depletion of phosphate ions from the regenerated scrubbing solution formed by contact between the pregnant scrubbing solution and the amine in the organic phase during the transfer reaction.
It will also be apparent from the described reduced selectivity of the organic transfer reaction, as a result of which a large portion of the organic phase extractant contains amine phosphates, that a greater amount of heat has to be applied to the total organic phase after occurrence of the transfer reaction, in order to release and drive off the relatively smaller quantity of sulfur dioxide which has been chemically combined with a given heated volume of the organic transfer reactant.
In another previously known SO.sub.2 removal process, the aqueous solution of sodium sulfite is used for scrubbing purposes. The pregnant scrubbing liquor, which is essentially an aqueous solution of sodium hydrosulfite, is then directly (without intervening processes) decomposed thermally to yield sulfur dioxide. In this method of proceeding, however, the heat input required to effect thermal decomposition of the hydrogen sulfite, as well as to vaporize a large quantity of water, is great. The water vapor thus emitted with sulfur dioxide must be subsequently condensed. The development of the large quantity of water vapor also complicates control of the decomposition process.
Another serious drawback of the described process involving direct decomposition of the sodium hydrosulfite is that at the temperature level at which decomposition of the aqueous hydrosulfite occurs, a portion of the hydrosulfite converts to sulfate. This conversion commonly yields from 5 to 10% sodium sulfate by-product, which represents a corresponding loss in the total sodium value required in the system for scrubbing purposes. In addition, removal of the sodium sulfate by-product is a complicated procedure involving several process steps.